#20
I'm going to rip 3 sheets of ply into 22" wide strips and laminate the 6 pieces together into a workbench top. I intend to get the flattest sheets I can find. Given the size of the sheets, I want to use curved cauls to apply pressure across the width of the glue-up in order to get the pressure as consistent as possible across the whole width of the top.

To this end, I have a couple of questions:


  1. Can I use plywood to make the cauls? I've got lots of plywood scraps left over from other projects that I can use.
  2. Failing using plywood, I have some red oak that I can use. Is that a good species?
  3. I was thinking of using a thin strip as a flexible guide to draw a fair curve on a template then cut them the material to the line & pattern route them to make them all identical. 
Thanks for any help you guys can give.

Tony
"The art of leadership is to work with the natural grain of the particular wood of humanity which comes to hand."

John Adair

My woodworking blog: Tony's Woodshop
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#21
If it were me, I would use plywood to make the cauls probably 3" x 4".  On a 22" glue up I would not have much curve in the caul.
"I tried being reasonable..........I didn't like it." Clint Eastwood
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#22
(06-20-2022, 08:26 AM)Bill Holt Wrote: If it were me, I would use plywood to make the cauls probably 3" x 4".  On a 22" glue up I would not have much curve in the caul.

Thanks, Bill. I can do that.

Now I gotta dig throw my plywood cutoffs.
"The art of leadership is to work with the natural grain of the particular wood of humanity which comes to hand."

John Adair

My woodworking blog: Tony's Woodshop
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#23
You will need to make a lot of cauls in order to even approximate uniform pressure on panels that large.  It would be better IMO to use sand bags or something similarly heavy to more uniformly distribute the force on the sheets.  I'd add an extra layer of plywood to the top of your stack, without glue, to help distribute the force from the sandbags, then cover the entire area with the bags.  

John
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#24
(06-20-2022, 09:36 AM)jteneyck Wrote: You will need to make a lot of cauls in order to even approximate uniform pressure on panels that large.  It would be better IMO to use sand bags or something similarly heavy to more uniformly distribute the force on the sheets.  I'd add an extra layer of plywood to the top of your stack, without glue, to help distribute the force from the sandbags, then cover the entire area with the bags.  

John

I like this idea.
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#25
I’d also be pretty careful to find or s h i m a stable surface very flat to glue it on. Weights are good to hold it down flat, but you need a lot of weight. A vacuum bag under the weights may not hold it as flat, but would add 30,000lb over that much area.

If you don’t have enough clamps, screws can work as clamps, but it takes a lot of screws and it’s a good idea to countersink the bottoms of the shank holes to allow surfaces to fully meet. Or bore holes to through bolt cauls on each face, and enlarge the holes into dog holes after drying.
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#26
Screw 2 sheets together in a pattern of squares 6x6.

When glue is dry remove screws, add second sheet and repeat.

Or, if you're topping it with hardboard, do it all at once

Easy peasy!!
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#27
Tony,

Before you make a lot of cauls out of plywood, make one and see if it works. The ends of a curved caul should touch the surface when you clamp it down so the caul is applying pressure along its entire length, not just the center. Since plywood is made to be dimensionally stable I question if it can work for this purpose. But then I can be wrong. Ask either of my ex wives.

Cliff
‘The problem with the world is that intelligent people are full of doubts, while the stupid ones are full of confidence
Charles Bukowski
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#28
Tony, I recently read an interesting method for making curved cauls that I'm going to try. I'll pass it on to you for your consideration.

Make the cauls out of 6/4 or 8/4 hardwooid stock. Cut them to length and width. Stand the caul blanks on edge and drill a hole to accommodate a stout bolt all the way through the center of each one. Counter bore one end of the holes to allow the head of the bolt to seat about 1/2" below the surface (edge) of the caul. Place shims beneath each end of the caul and bolt it to your bench top (or any stable work surface). Tighten the bolt until the center of the caul touches the surface of the workbench, making sure the shims remain in place under each end. This will flex the caul blank introducing a curve into it. With a sharp hand plane, plane the concave top of the caul flat. Loosen and remove the bolt. You should have a nicely curved caul. The curve is the natural shape of the caul blank flexed under pressure. This should guarantee even pressure on your glue surface when you clamp and tighten both ends of the caul onto your glue-up. You don't need much curve, so your shims should be of modest thickness. The longer the caul, the more curve (thicker shims) you need.
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#29
The point of a curved caul is just to get clamping pressure in the middle. But you want to make sure it’s all flat, so you either need to carefully balance opposing cauls, or make sure one is much more rigid than the other and straight.

A much easier approach that can work for workbenches is to use straight cauls (edge jointed 2x4?) bolted through the plywood in the middle. Obviously this isn’t appropriate if you can’t have a hole there, but dog and holdfast holes are useful.
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Making Cauls for laminating plywood


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